What’s Your “Odor Print”? Delving Into The Genetics Of Olfaction

Contributors: Casey Trimmer and Nicole Greenbaum, Monell Chemical Senses Center, Pennsylvania, USA.

Target audience: Ages 14 and older.

Background: We sense odors with detectors in our nose called olfactory receptors. We have over 400 types of detectors in our nose, which are activated by odors in unique combinations to give us information about what we’re smelling—what an odor is, how strong it is, and whether we like it. This demonstration illustrates how inborn variation in our olfactory receptors can affect the way they function, changing our perception of an odor. These phenomena are similar to color blindness in color vision, where genetic changes in a vision receptor result in perceptual alterations.

Genetic variation is common in the olfactory receptor gene family. While “natural knock-outs,” where genetic changes render a gene non-functional, are rare in the genome, they’re quite common in olfaction. On average two people will differ functionally at ~30% of their ~400 recep-tors, meaning everyone experiences their olfactory world differently. Therefore, just as every individual has different fingerprints, every individual has a “nose print” that is unique to them. To illustrate this point, in this demonstration participants smell a panel of odors and indi-cate whether they can smell it and if they like it. Four odors is generally enough to discriminate among participants, i.e. people will have a fairly unique “nose print” using only four odors. After participants finish filling in the squares on their cards (image included below), the cards are dis-played so everyone can compare, and see how different people’s noses are. Using odors common in food, beverages, and personal products ties this variation to real-life and shows people how this variation affects the way we interact with the world around us.

Goal(s): To illustrate that, just as genetic differences affect our height and eye color, they also influence the way we perceive odors, including our ability to detect an odor or how strong or pleasant we find it.

Materials:

This demonstration works best with specific chemical molecules rather than household items. We list examples of more common products that contain the molecule below, but it may be worth purchasing the pure odorants themselves to maximize the demonstration’s effect (see the NOTE below). Recommended odors (and more commonly found products that contain them) include:

1. Beta-ionone has the characteristic odor of violets. For some (~50%), this odorant has a very strong floral quality, while others get a faint “sweet” note or are completely unable to smell it. (FYI: It’s a dramatic 50/50 split, and, if needed, a simplified variation of this demonstration can be done with this odor alone – some will smell a strong floral odor and some will smell an empty jar from the same jar. It’s not as convincing as the full card of four odors, but it can serve reasonably well on its own as an example of genetic variation leading to perceptual differences.) Beta-ionone is a common component of perfumes and foods and beverages, such as pinot noir and various fruits such as raspberries and mangos. The ability to smell this odorant is related to genetic variation in OR5A1. Suggested dilution of pure odor: 0.2% in paraffin oil. Beta-ionone can be found in violet essential oil.

2. Guaiacol is a smoky odor found in bacon, beef jerky, smoked fish, and peaty scotch. The majority of the population can smell guaiacol to some degree, but people vary in their sensitivity to it. In addition, guaiacol’s pleasantness is inversely related to sensitivity, i.e. the more intense it is, the less pleasant you tend to find it. So, while most will fill in the box for “Do you smell it?” for guaiacol, they usual vary in the “Do you like it?” column. This phenomenon is related to genetic variation in the olfactory receptor OR10G4. Suggested dilution of pure odor: 0.1% in paraffin oil. Guaiacol can be found in Liquid Smoke.

3. 2-Ethylfenchol has an earthy quality. It is characteristic of root vegetables such as beets and potatoes and oolong tea. People vary in their ability to detect it, with roughly 30% of the population either less sensitive or completely unable to smell it (Trimmer et al., 2017). This phenomenon is related to genetic variation in olfactory receptor OR11A1. Suggested dilution of pure odorant: 0.1% in paraffin oil. 2-Ethylfenchol can be found in beet essential oil or oolong tea.

4. Galaxolide is a synthetic musk. Musks have a long history of use—thousands of years—and are commonly added to perfumes as long-lasting base notes. Roughly 12% of the population is unable to smell galaxolide (Knaapila et al., 2012), although this does not mean they are unable to smell other musks. The olfactory receptor associated with variation in galaxolide perception is still unknown. Suggested dilution: 10% in paraffin oil. Galaxolide can be found in eau de cologne.

5. Androstenone is perceived as unpleasantly sweaty and urinous by roughly 30% of the population. The remainder of the population either finds it to be fairly pleasant (like vanilla, sandalwood, or alcohol) or completely odorless. Variability in androstenone perception is associated with genetic variation in a single olfactory receptor OR7D4. Suggested dilution of pure odor: 0.1% w/v in paraffin oil. Androstenone can be found in human sweat.

Opaque jars with absorbent material on the bottom. Add a small amount (approximately 0.5 milliliters) of odor dilution or product to jar.

NOTE: As mentioned above, if you decide not to buy pure odors, the effect will be less impactful. Participants with genetic variation that affects the relevant receptor will experience products like violet oil and Liquid Smoke differently, but, because these products are mixtures, participants will still be able to smell the product, so all will likely mark the box in the “Can you smell it?” column. For information on how to buy the chemicals listed here, please contact [email protected] including in the subject of your email the text: “Odor print demo”.

Steps:

• Present to participants one odor at a time;
• Ask them to evaluate whether they can smell it;
• If they can smell it, ask them if they like it;
• Compare the “odor prints” from multiple people.

Take-home message: Inborn variation in our olfactory receptors can affect the way they function, changing our percep-tion of an odor. On average two people will differ functionally at roughly 30% of their 400 receptors, meaning everyone experiences their olfactory world differently and has a “nose print” that is unique to them.

If you want to read up more on this topic try these scientific papers:
Wysocki, C.J., and Gilbert, A. N. (1989). National Geographic Smell Survey. Effects of age are heterogenous. Ann. N. Y. Acad. Sci. 561, 12–28.

Keller, A., Zhuang, H., Chi, Q., Vosshall, L.B., and Matsunami, H. (2007). Genetic variation in a human odorant receptor alters odour perception. Nature 449, 468–472.

Jaeger, S.R., McRae, J.F., Bava, C.M., Beresford, M.K., Hunter, D., Jia, Y., Chheang, S.L., Jin, D., Peng, M., Gamble, J.C., et al. (2013). A Mendelian Trait for Olfactory Sensitivity Affects Odor Experience and Food Selection. Curr. Biol. 1–5.

Mainland, J.D., Keller, A., Li, Y.R., Zhou, T., Trimmer, C., Snyder, L.L., Moberly, A.H., Adipietro, K. a, Liu, W.L.L., Zhuang, H., et al. (2014). The missense of smell: functional variability in the hu-man odorant receptor repertoire. Nat. Neurosci. 17, 114–120.

Trimmer, C., Keller, A., Murphy, N.R., Snyder, L.L., Willer, J.R., Nagai, M., Katsanis, N., Voss-hall, L.B., Matsunami, H., and Mainland, J.D. (2017). Genetic variation across the human olfacto-ry receptor repertoire alters odor perception. bioRxiv, 212431.